The present invention generally relates to power MOSFET transistors and more particularly to power MOSFET transistors having a variety of mesa dimensions.
The use of power MOSFETs is quickly becoming ubiquitous and their popularity is sure to spread in the coming years as they are called upon to serve in an increasing number of applications. But the demands of these applications are placing a burden on the performance of these devices. Accordingly, power MOSFET devices having improved performance are needed.
During typical operation, when conducting, a power MOSFET pulls current through an inductor. When the power MOSFET turns off, the inductor retains a stored energy. This stored energy generates a current though the inductor that decreases over time as a function of the quality factor or “Q” of the inductor. The current is dissipated in the power MOSFET as that device enters the avalanche breakdown mode.
Avalanche current passes through a power MOSFET device via its body diode, that is, the junction between its drain and well regions. For an n-channel transistor, avalanche current flows from an N-type drain, through a P-well, to the heavy-body contact. Since the transistor is designed to handle this current, this action is not destructive.
However, if the well-to-source diode turns on, a secondary parasitic bipolar action can begin. This secondary bipolar action can lead to a runaway current that can be destructive to the device. The well-to-source diode can turn on if the well resistance is excessive, that is, if the pinched-base resistance is too high.
Accordingly, it is desirable to reduce the pinched base resistance to prevent the secondary bipolar action. But many techniques of reducing this resistor result in a larger cell size. A larger cell size reduces the number of cells that can be integrated and reduces device current carrying capacity.
Thus, what is needed are circuits, methods, and apparatus that provide power MOSFET transistors having a large number of cells while maintaining a low pinched-base resistance.